This invention relates generally to thermal ink transfer printing apparatus, and particularly to such apparatus which is capable of selecting a desired tone value for each picture element.
Thermal ink transfer printing, whose principle is shown in FIG. 1, is arranged such that a print sheet 1 and an ink transfer sheet contacting each other are sent to a portion between a platen roller 3 and a thermal head 4, and when the ink transfer sheet 2 is heated with respective heat generating resistors of the thermal head 4 being operated to generate heat by applying currents to the respective heat generating resistors, thermally fusable ink 2' painted on the ink transfer sheet 2 at a constant thickness is molten to be transferred to the print sheet 1, and then the print sheet to which ink transfer printing has been effected is cut to have a suitable shape so as to be a printed matter, while the ink transfer sheet which has become unnecessary is taken up by a take up roller via a guide roller so as to be disposed.
Printing with a desired tone value, which will be simply referred to as tone value printing hereinafter, in such thermal ink transfer printing is effected such that a given area of ink on the ink transfer sheet is molten by way of a heat generating resistor so as to obtain a given density, namely, area control is effected such that when density is low, the area to be molten is small and when density is high, the area to be molten is large.
If the area of each heat generating resistor is made small so as to obtain a fine printing result, since maximum area cannot be made large on tone value printing, accurate density indication cannot be performed because white-omission occurs when it is required to perform uniformly-density printing over a relatively large area. When power is made large so as to obtain a density or tone value similar to uniform-density printing, then there is a drawback that the heat generating resistor burns out. Furthermore, there is a problem that linear printing cannot be performed since the input signal and temperature rise are not in linear relationship.
On the contrary to the above, when the area of each heat generating resistor is made large, a minimum area is necessarily made large resulting in coarse printing.